1. Field of the Invention
The present invention relates to an integrated circuit device and an audio system.
2. Background Art
In many cases, an audio system is provided with a muting circuit for muting an audio signal, and capable of switching between a sound output state and a mute state. It is known that a rasping sound referred to as “pop sound” is induced in such an audio system, when switching between the sound output state and the mute state. To reduce this pop sound, such an audio system is often provided with a time constant circuit for slowing the switching between the sound output state and the mute state.
The time constant circuit is generally configured with a resistor R and a capacitor C. A time constant around 10 to 100 msec is considered necessary to reduce the pop sound. However, it is difficult to achieve such a time constant with an IC (integrated circuit). Therefore, in the audio system, though the muting circuit is generally realized with the IC, the time constant circuit is provided out of the IC. In this case, the time constant circuit is connected to a time constant control terminal of the IC.
In recent years, attention has been attracted to multi-CH (channel) independent muting. For example, there exists such a demand that only a specific CH should be muted among six CHs of an audio system. In such a case, time constant circuits and time constant control terminals as many as CHs are required. However, as described above, since a time constant circuit is realized with elements out of an IC, it is highly costly to provide the time constant circuit. Therefore, it is disadvantageous in terms of cost to provide time constant circuits as many as CHs. Therefore, an audio system is proposed in which a single time constant circuit and a single time constant control terminal are associated with plural CHs (JP-A 2005-295011 (KOKAI) for example).
Description will now be made to an example of the audio system in which a single time constant circuit and a single time constant control terminal are associated with plural CHs.
Such an audio system includes, for example, sound output amplifiers and mute amplifiers, and a muting circuit of the audio system outputs mute control currents for controlling the sound output amplifiers and mute control currents for controlling the mute amplifiers, to mute an audio signal. The muting circuit outputs mute control currents for a first sound output amplifier and a first mute amplifier for muting a first CH, and outputs mute control currents for a second sound output amplifier and a second mute amplifier for muting a second CH. The muting circuit is connected to the time constant control terminal and the time constant circuit, both of which are common to these CHs.
An exemplary configuration of the muting circuit will be described. The muting circuit includes plural mute control current generating circuits connected to the time constant control terminal, a charging and discharging circuit configured to charge and discharge the time constant control terminal, and a control circuit configured to control the plural mute control current generating circuits and the charging and discharging circuit. Each of the mute control current generating circuits includes a V-I (voltage to current) converting circuit configured to compare a voltage of the time constant control terminal with a intermediate voltage, and generate a current IA corresponding to the voltage of the time constant control terminal and a current IB corresponding to the intermediate voltage, and a mute control current selecting and combining circuit configured to selectively combine the current IA and the current IB to generate a current Iout. In a subsequent stage of each of the mute control current generating circuits, there are provided a resistor for converting the current Iout to a voltage Vout, and a V-I (voltage to current) converting circuit for generating mute control currents Iout-A and Iout-B by using the voltage Vout (the V-I converting circuit is also referred to as a Gm amplifier, and hereinafter, may be expressed as a subsequent V-I converting circuit). The mute control current generating circuits are provided as many as the CHs of the audio system.
Then, an example of operations of the muting circuit will be described. Under the control by the control circuit, the charging and discharging circuit starts charging the time constant control terminal. The time constant control terminal is continued to be charged until the voltage on the time constant control terminal is changed from a first reference voltage to a second reference voltage. Since the time constant control terminal is connected to the time constant circuit, the time constant control terminal is slowly charged. It is assumed that a period for charging the time constant control terminal is t1. The V-I converting circuit needs to switch between a value of IA and a value of IB during t1, and is designed so as to carry out such a switching. Therefore, assuming that this switching period is t2, a relation between t1 and t2 becomes t1>t2. The subsequent V-I converting circuit needs to switch between a value of Iout-A and a value of Iout-B during t2, and is designed so as to carry out such a switching. Therefore, assuming that this switching period is t3, a relation between t2 and t3 becomes t2>t3.
The period t3 corresponds to a time required for switching between the sound output state and the mute state. Therefore, a shorter period t3 causes rapid switching between the sound output state and the mute state, so that an effect on the pop sound reduction is reduced. Accordingly, it is desirable that the period t3 is as long as possible. However, since there exists such a restriction that a relation between t1, t2, and t3 is t1>t2>t3, it is difficult to extend the period t3. Due to this restriction, it is necessary to extend the charging period t1 in order to extend the period t3. In order to extend the charging period t1, it is necessary to make larger a time constant of the time constant circuit by means of a large capacity of condenser or the like. However, this results in the increased cost of the time constant circuit. As described here, it is difficult to sufficiently reduce the pop sound in the above muting circuit.
Further, the above audio system requires the mute control current Iout-A for the sound output amplifier, and the mute control current Iout-B for the mute amplifier. Therefore, the muting circuit requires the subsequent V-I converting circuit for generating the mute control currents Iout-A and Iout-B from the current Iout (the voltage Vout). Therefore, the muting circuit would suffer from a problem of an increased IC chip area due to including the subsequent V-I converting circuit.